as far as i know a dead spot is a place on your bass's fingerboard where the resonance and bottom end of the note you are playing is just gone, the spot will sound much different from others on the fingerboard;
its dead because it's (meaning the location on the fingerboard"or fretboard") sound lacks the qualities of the other "fretted" notes

A dead spot is a note on the neck that lacks in sustain in comparison with other notes on the neck. Dead spots are most commonly found on the G string, typically between C and D. I have a MIM Jazz, however, that has a dead spot at about D#.

Having dead spots doesn't mean the note doesn't ring at all. That seems to be a common misconception around here. Instead, if you play a note that's on a dead spot, what you'll notice is a much shorter sustain than other places on the neck, particularly in the fundamental. On my jazz, when I play the D#, the attack is clearly present, however the note decays very quickly, and after just a few moments, the only tone left is harmonic several octaves up. This is an extreme example of a dead spot.

It can be argued that all wood-neck basses have dead spots. My Spector doesn't seem to have any. However, dead spots sometimes come BETWEEN notes, meaning unless my bass was fretless, I'd probably never recognize the dead spot. Of course, it's quite possible the graphite stiffening rods in the neck help